Synthetic thermoplastic resin films such as those of polyesters, polyamides, polypropylene and the like, particularly, polyester films, inter alia, polyethylene terephthalate films are useful in various fields such as electrical use, ornamental use, packaging materials, magnetic tapes and the like because they have excellent mechanical properties, electrical characteristics and dimensional stability. However, it has hitherto been well known that, when a friction coefficient of a thermoplastic resin film, particularly, a polyester film is high, slipperiness between the films becomes poor and, when slipperiness is extremely inferior, it causes blocking and makes not only handling of the film but also a film forming operation, particularly, winding of the film difficult. In order to lower a friction coefficient of the film, an inorganic or organic compound alone or a combination thereof is added to the film. However, when an amount of the compound is small, the effect thereof is hardly obtained. Therefore, the compound should be added to the film in a relatively large amount, which results in remarkable decrease of transparency of the film. That is, it has not been found heretofore in the prior art the film having a significantly lowered friction coefficient as well as good transparency as that of the film obtained without addition of such an compound. Further, when the same amount of the same compound is added to a polyester, the degree of lowering of a friction coefficient varies depending upon conditions of heat treatment. When, thermal history is higher, this degree of lowering of a friction coefficient becomes greater.
On the other hand, it has been proposed that a film having good slipperiness and good transparency can be obtained by adding an organic polysiloxane to a polyester film. According to this method, slipperiness of the film increases with increasing amount of an organic polysiloxane but, contrary to this, transparency of the film decreases and dimensional stability and mechanical properties such as Young's modulus, etc. tend to be inferior. Further, a method wherein knurl treatment is effected before winding a film has been also proposed. However, in fabrication of the film, the knurl treated part is cut off by a slit or remained only on one side. Thus, there are various problems in these conventional methods.
Further, common polyester films have high electrical insulation properties and therefore they have such drawbacks that they tend to generate and build up static electricity which cause various troubles due to static hindrance. For example, in film formation, printing, adhesion, bag making, packaging and other fabrication, static electricity causes lowering of operating efficiency (e.g., roll retention of the film, electric shock to the human body and difficulties in handling) and lowering of commercial value of end products (e.g., print having whisker and dirt on film surfaces). In general, as a method for preventing such a static hindrance, there is employed a treatment wherein an antistatic agent is incorporated into a resin to form a film or applied on a surface of a film. However, with respect to a polyester film, it is difficult to effect the former so-called incorporation type antistatic treatment. That is, in this treatment, an antistatic agent should ooze out from the inside to the surface of a film to show the antistatic effect, while, in case of a polyester film, once a film has been formed, an antistatic agent can not ooze out to the surface of the film at about room temperature because a polyester resin has a high second-order transition temperature. In addition, because of high film forming temperature conditions and high reactivity of a polar group contained in a polyester per se, there exist various problems in the addition of an antistatic agent such as deterioration of a polymer during film formation, coloring and lowering of physical properties. Particularly, in case of a biaxially oriented polyester film, the antistatic effect can be hardly expected because an antistatic agent on the surface of the film is scattered and lost during orientation. Further, most of antistatic agents cause remarkable decrease of transparency when they are added to a polyester film and they are difficult to use in practice.
In addition, it is very rare to use a biaxially oriented polyester film as it is in various uses and, usually, an appropriate surface treatment is provided to the film according to a particular use, for example, to improve adhesion between the film and a gelatin layer in case of using for a photographic film base, between the film and a magnetic layer in case of using for a magnetic tape base, between the film and a matting agent layer in case of using for a drafting base, between the film and a deposited metal in case of deposition of a metal, or between the film and an ink mainly composed of a nitrocellulose binder or a heat sealing agent in case of using for packaging. However, when a primer is used to improve adhesion, there is such a problem that, in general, a primer having good affinity for a biaxially oriented polyester film shows poor adhesion to a surface layer, while a primer having good affinity for the surface layer shows poor adhesion to the film. Further, as a method for improving adhesion properties of a surface of a polyester film, corona discharge treatment is generally employed and, in addition to this, a physical treatment such as ultraviolet-light irradiation treatment, plasma irradiation treatment, flame treatment, corona discharge treatment under nitrogen atmosphere or the like, a chemical treatment such as a treatment with an alkali, a primer or the like, or a combination thereof is known. However, it has not been found heretofore in the prior art a polyester film having good adhesion properties to both hydrophilic and hydrophobic polymers.
Under these circumstances, in order to obtain a thermoplastic resin film, particularly, a polyester film having excellent slipperiness and transparency and excellent other necessary properties such as antistatic properties and adhesion properties, the present inventors have intensively studied. As the result, it has been found that the desired thermoplastic film can be obtained by providing a layer of a polyester resin composition comprising a specific copolyester and inert particles and, optionally, one or more ingredients selected from the group consisting of a water soluble silicone, a polyfunctional reactive compound, a polyethylene glycol and its derivative and an antistatic agent to at least one surface of a thermoplastic resin film base, particularly, a polyester resin film base.